1. Field of the Invention
This invention relates to a bilateral amplifier.
2. Description of the Prior Art
Various kinds of bilateral amplifier which can amplify not only in one direction but also in the other direction are known and in practical use. One such type of bilateral amplifier uses a hybrid transformer or the like and operates on the basis of a 2-way/4-way separating section. In another type of bilateral amplifier, negative impedance is used.
FIG. 1 shows the principles of the former type of bilateral amplifier. A signal produced from a signal source 10 (which may be a transmitting section of a telephone set) is input through a load 11 to one pair of input terminals 1c and 1d of a 2-way/4-way separating section 12 and output from a pair of output terminals 1e and 1f of the section 12 to be amplified in an amplifier 13. The output of the amplifier 13 is input to a pair of input terminals 2a and 2b of another 2-way/4-way separating section 22 and output from a pair of input terminals 2c and 2d to be consumed in a load 21 (which may be a receiving section of a remote telephone set). Meanwhile, a signal produced from a remote side signal source 20 is input through the load 21 to the pair of input/output terminals 2c and 2d of the other 2-way/4-way separating section 22 and output from a pair of output terminals 2e and 2f of the section 22 to be amplified in another amplifier 23. The output of the amplifier 23 is input to another pair of input terminals 1a and 1b of the 2-way/4-way separating section 12 and output from the pair of input terminals 1c and 1d to be consumed in the load 11.
In this system, in order to ensure sufficient degree of separation of the two 2-way/4-way separating sections 12 and 22, impedance circuits 14 and 24, the impedance of which is equal to the impedance of the loads 11 and 21 connected to the respective pairs of input/output terminals 1c and 1d and 2c and 2d, must be connected to respective further pairs of terminals 1g and 1h and 2g and 2h of the separating sections 12 and 22. Where the impedance of the impedance circuits 14 and 24 and the impedance of the loads 11 and 21 are not equal, the separation performance of the 2-way/4-way separating sections 12 and 22 is deteriorated. More specifically, the signal input to the terminals 1a and 1b does not output 100% from the terminals 1c and 1d but partly leaks to the terminals 1e and 1f. Likewise, a signal input to the separating section 22 partly leaks to the terminals 2e and 2f. In such a case, feedback loops are established with respect to the amplifiers 13 and 23, thus reducing the degree of amplification and also leading to such undesired phenomena as oscillation. To avoid these undesired effects, the degree of amplification must be reduced. Where the loads 11 and 21 are pure resistances, the impedance circuits 14 and 24 may be simple, and the degree of reduction of the amplification as noted can be avoided. Where the loads 11 and 21 are distributed constant circuits represented by an equivalent circuit as shown in FIG. 2, consisting of infinite numbers of progressively connected L, C, R and G, however, it is difficult to construct an identical distributed constant circuit. In this case, therefore, an equivalent circuit is realized with a lumped constant circuit. To this end, however, an active complicated circuit construction is required. If it is difficult to realize such a circuit, the afore-mentioned reduction of degree of amplification results. Further, where the loads 11 and 21 are indefinite, it is impossible to design the equivalent impedance circuits 14 and 24. Further, the bilateral amplifier utilizing a hybrid transformer requires two separating sections, which is undesired from the standpoint of economy.
FIG. 3 shows the principles of the latter type of bilateral amplifier, i.e., the one which makes use of negative impedance.
The amplifier includes negative impedance converters 32 and 42 (hereinafter referred to as NIC). The NIC 32 forms together with an impedance circuit 33 a current type negative impedance. The NIC 32 is connected in series with loads 31 and 41. The NIC 42 forms together with an impedance circuit 43 a voltage type negative impedance. The NIC 42 is connected in parallel with the loads 31 and 41, and it is connected to a mid point of a coil 35 connected in parallel with the NIC 32.
Designated at 30 and 40 are signal sources, and at 31 and 41 are loads which consume signals transmitted from the respective signal sources 30 and 40.
It is well known that impedance matching is obtained when relations ##EQU1## are met, where ZL is the impedance of the loads 31 and 41, Zs and Zp are the impedances of the respective impedance circuits 33 and 43, and N is any positive real number, and also the degree of voltage amplification .gamma. is expressed as ##EQU2##
The latter equation means that .gamma. becomes infinity as N approaches 2. In order to increase the degree of amplification, therefore, it is necessary to let Zp approximate ZL/2 and let Zs approximate 2ZL. If these approximations for the impedance circuits 33 and 43 are unsatisfactory, the degree of amplification may be unsatisfactory, or the oscillation phenomenon is liable to result. That is, with this type of bilateral amplifier, like the previously described type, if the equivalent circuits of the loads are hard to realize or complicated, it is necessary to increase the amplification degree due to low amplification or to avoid the oscillation phenomenon. Further, this type of bilateral amplifier requires two NICs and two impedance circuits, which is disadvantageous in view of economy.